How big is baby?

Your due date will be calculated from the first day of your last menstrual period. Conception occurs about two weeks from this day, and that's when you're truly considered pregnant. In just 40 short weeks, your baby will grow from the size of a tiny seed to the size of a plump watermelon.

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Fetus at Week 1What the baby looks like at week 1 of fetal development. Med ART Studios Explore More

Baby's Development

Baby? What baby? This concept may be a little hard to wrap your head around, but even though you're technically in the first week of pregnancy, you're not quite expecting -- yet. Here's the deal: Because it's generally impossible to know the exact moment of conception, most healthcare providers count 40 weeks from the first day of your last menstrual period (LMP) to calculate your due date. According to this method, they date the beginning of "pregnancy" from about two weeks before the sperm penetrates the egg -- which is where you are right now. Already spotted that pink line on a home pregnancy test? You're further along than you think, so skip ahead to Week 4.

Baby Week 1

Week 1 Ultrasound

There's no ultrasound image of your baby-to-be for weeks 1 and 2. While your health care provider counts these two weeks toward your due date, you aren't really pregnant. Confused? Your pregnancy due date is calculated using the first day of your last menstrual period (LMP). Obviously you weren't pregnant at that time, but it's the best reference your health care provider has for estimating baby's arrival day (until you get an ultrasound, which may provide a more accurate due date).

Watch Baby's Growth

Sperm Meets Egg: Weeks 1 to 3 of Pregnancy

Mom's changing body

If you're trying to conceive (or think you might already be pregnant), start making health changes to ensure that your body is in the best shape for carrying and nurturing your baby. The biggies: cut out alcohol, cigarettes, and any illegal or recreational drugs. You may also need to switch or stop taking some prescription and over-the-counter medications. So tell your doctor that you're trying to conceive and make sure any medications you're using are safe.

Weekly Tip

Speak with your doctor about adding a prenatal multivitamin to your morning routine. At least 400 micrograms of folic acid every day can help prevent certain birth defects that affect your baby's brain, heart, and spinal cord, especially if you take it before you conceive and in the early weeks of your pregnancy. See that your supplements contain added goodies like calcium, iron, and Vitamin B12 as well.

If you're like many women, you're probably still a little mystified by your cycle, and you probably haven't had to think twice about it until you decided to start a family. Once you throw conception into the mix, things can get even trickier to understand. So we asked two top doctors to answer some of the most common questions women have about their reproductive health.

What's going on with your baby when you're one and two weeks pregnant? Find out all about important pregnancy milestones, symptoms and exciting fetal development specific to the start of your baby's journey!